1. Field of the Invention
The present invention relates to a reversible thermosensitive recording label and a reversible thermosensitive recording card which are capable of recording and erasing images repeatedly by utilizing the properties of the transparency thereof which reversibly changes from a transparent state to an opaque state, and vice versa, depending upon the temperature thereof.
2. Discussion of Background
Recently attention has been paid to a reversible thermosensitive recording material capable of temporarily recording images thereon and erasing the same therefrom when such images become unnecessary. As representative examples of this kind of reversible thermosensitive recording material, there are conventionally known reversible thermosensitive recording materials in which an organic low-molecular-weight material such as a higher fatty acid is dispersed in a matrix resin such as vinyl chloride-vinyl acetate copolymer with a glass transition temperature (Tg) of as low as 50.degree. C. to less than 80.degree. C., as disclosed in Japanese Laid-Open Patent Applications 54-119377 and 55-154198.
Moreover, the following reversible thermosensitive recording methods have been proposed: a method of displaying recorded information on a reversible thermosensitive recording card provided with a colored portion on the back surface thereof as disclosed in Japanese Laid-Open Utility Model Application 2-3876; a method of displaying recorded information on a reversible thermosensitive recording material provided with a light reflection layer on the back surface thereof for improving image contrast of the displayed information as disclosed in Japanese Laid-Open Patent Application 64-14079; and a method of providing a reversible thermosensitive recording material with a thin film layer at the back surface thereof with a different refractive index from that of a recording layer thereof as disclosed in Japanese Laid-Open Patent Application 2-175280.
In addition, the formation of a reversible thermosensitive recording material comprising a protective layer has been proposed for the purpose of protecting the recording layer from thermal hysteresis. This kind of protective layer can be made of a silicone rubber or silicone resin as disclosed in Japanese Laid-Open Patent Application 63-221087, a polysiloxane graft polymer as disclosed in Japanese Laid-Open Patent Application 63-317385, or an ultraviolet-curing resin or electron radiation curing resin as disclosed in Japanese Laid-Open Patent Application 2-566.
However, the above-mentioned reversible thermosensitive recording materials have the shortcomings that they are caused to deteriorate by the repetition of image recording and erasure under the application of heat and pressure by use of heat-application means such as a thermal head.
Recently, a demand is for displaying information on a thick card made of a material such as polyvinyl chloride which is used as an IC card, an ID card or a cash card. However, it is extremely difficult to uniformly coat a coating liquid for a reversible thermosensitive recording layer on a thick card made of polyvinyl chloride because (1) the polyvinyl chloride in the card is dissolved by a solvent contained in the coating liquid for the reversible thermosensitive recording layer and (2) it is extremely difficult to uniformly coat the coating liquid for the reversible thermosensitive recording layer on a thick and rigid card by means of a coating machine.
In the case of a thick card type reversible thermosensitive recording material, there is the risk that non-printed portions are formed on the images obtained unless image formation is performed under the application of a sufficiently high pressure by a thermal head thereto. Therefore, it is necessary to increase the pressure applied by the thermal head. However, when the pressure of the thermal head is set high, scratches tend to be formed on the surface of the recording material, and when the reversible thermosensitive recording material is in the form of a label, the label is easily peeled off a label receiving material.
Furthermore, when such a reversible thermosensitive recording material comprises a printed layer on the surface thereof, excellent printing suitability can be initially obtained. However, when the above reversible thermosensitive recording material is subjected to image formation and erasure many times by use of heat-application means such as a thermal head, the printed layer is scraped off by the heat and pressure applied thereto by the thermal head or by transportation rollers in a recording apparatus.
In particular, when a reversible thermosensitive recording card comprising a recording material applied to a thick card serving as a label receiving material is used for image formation, the pressure applied to the card by a thermal head is high as mentioned previously, and a large force is applied thereto in a transportation unit of a recording apparatus, so that the printed layer is considerably scraped and the reversible thermosensitive recording label tends to be peeled off the label receiving material in the transportation unit.
Conventionally, in a label employed for recording by use of a thermal head, silicone paper is employed as a disposable release sheet for the label and a pressure-sensitive adhesive agent is employed for applying the disposable release sheet to a recording member of the label. A representative example of such a pressure-sensitive adhesive agent is an acrylic emulsion.
When this type of label sheet is employed repeatedly, which is not made for repeated use and whose adhesion to a label receiving material is designed so as to be sufficient only for a single use, there is the risk that the label is peeled off the label receiving material in the course of repeated use through a transportation unit of a recording apparatus.
In the case where the adhesive agent for the above-mentioned label is replaced by an adhesive agent which can impart sufficient durability to the label for the repeated use thereof, it is possible to prevent the peeling of the label off the label receiving material during the transportation through a recording apparatus, but other problems such as the formation of non-printed portions in the obtained images occur when the label receiving material is thick and deformed.